Touch-sensitive devices may detect touch-based inputs via several different mechanisms, including but not limited to optical, resistive, acoustic, and capacitive mechanisms. Touch-sensitive devices that are capable of sensing multiple touches are increasing in prevalence in the industry.
Capacitive sensors are one of the more cost effective and durable techniques for sensing touch input. A capacitive sensor may be fabricated with a deposition of ITO (Indium Tin Oxide) on glass, plastic, or another substrate. In a capacitive sensor designed for sensing multiple touch points, the ITO is deposited in a fine array of horizontal and vertical lines on different surfaces that form a grid. In this manner, capacitance values may be measured at the intersection of each of the lines forming the grid. Touching at or near the intersection of one or more lines changes the capacitance in the area of the intersection and in turn changes in capacitance can be sensed by supporting electronic circuitry and the touch points can be located algorithmically. Other types of analog capacitive sensors have a single unbroken coating of ITO and oscillators connected to each corner. When touched by a human finger, changes in the relative oscillator frequencies allow computation of the touch point to a high degree of accuracy.
Capacitive touch sensors have constraints in screen resolution and size. These constraints are in part due to electronic scanning of the ITO lines to measure capacitance values because it takes a fixed measurement time to achieve a certain precision. Further, increasing ITO line density for resolution or scaling ITO lines to a larger screen size results in a larger number of lines to be scanned in a similar time frame, reducing the duty cycle of sensing per line and also reducing scan time per each line. Higher density or longer lines also results in increased crosstalk and signal loss. Additionally, accuracy in reading capacitance values is related to measurement time, therefore capacitance readings become less accurate as touch screen sizes or densities increase. Variability in the ITO deposition process may further reduce accuracy in capacitance value measurements.